Plaques of subpial demyelination were induced in adult cat spinal cords by repeated withdrawal and reinjection of cerebrospinal fluid. Peripheral cord was fixed by replacing cerebrospinal fluid available at cisternal puncture with 3 per cent buffered OsO4. Following extirpation, surface tissue was further fixed in 2 per cent buffered OsO4, dehydrated in ethanol, and embedded in araldite.

Normal subpial cord consists mainly of myelinated axons and two types of macroglia, fibrous astrocytes and oligodendrocytes. Twenty-nine hours after lesion induction most myelin sheaths are deteriorating and typical macroglia are no longer visible. Phagocytosis of myelin debris has begun. In 3-day lesions, axons are intact and their mitochondria and neurofibrils appear normal despite continued myelin breakdown. All axons are completely demyelinated by 6 days. They lack investments only briefly, however, for at 10 and 14 days, macroglial processes appear and embrace them. These macroglia do not resemble either one of the normally occurring glia; their dense cytoplasm contains fibrils in addition to the usual organelles.

It is proposed that these macroglia, which later accomplish remyelination, are the hypertrophic or swollen astrocytes of classical neuropathology. The suggestion that these astrocytes possess the potential to remyelinate axons in addition to their known ability to form cicatrix raises the possibility of pharmacological control of their expression.

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